As a common mode choke coil whose perimeter is covered by a box-like magnetic core, a common mode filter disclosed in Japanese Unexamined Patent Application Publication No. 2003-243228 is known, for example. This type of common mode filter includes a pot-type core which is a box-type magnetic body whose one side is an opening portion, a flat plate magnetic body configured to seal the opening portion, and two helical coils positioned inside the pot-type core. The two coils are wound so that the central axes thereof match each other, and conductive wires configuring the respective coils are so provided as to be alternately laminated on each other.
In the case where a common mode alternating-current (AC) current flows in the common mode filter disclosed in Japanese Unexamined Patent Application Publication No. 2003-243228, directions of magnetic fluxes generated by the current flowing through the two coils are the same. Therefore the generated magnetic fluxes strengthen each other; as a result, the common mode filter functions as an inductor. On the other hand, in the case where a normal mode AC current flows in the common mode filter, the directions of the magnetic fluxes generated by the current flowing through the two coils are opposite to each other. Therefore the generated magnetic fluxes cancel each other out; as a result, the common mode filter does not function as an inductor.
In general, power supply lines of electronic devices, motor devices, and the like include not only common mode noise components but also normal mode noise components. As such, there is a requirement for common mode filters to additionally have a function of reducing the normal mode noise. However, even if the common mode filter disclosed in Japanese Unexamined Patent Application Publication No. 2003-243228 is intended to function as an inductor against a normal mode AC current, the generated magnetic fluxes cancel each other out, as discussed above, so that it is difficult for the stated common mode filter to function as an inductor.